In recent years, semiconductor thin film photoelectric conversion devices as represented by a solar cell have been diversified, and crystalline silicon thin film solar cells have been developed in addition to conventional amorphous silicon thin film solar cells. Furthermore, a tandem (hybrid)-type thin film solar cell having a stack thereof have come into practical use.
In general, a silicon thin film photoelectric conversion device includes a first electrode, one or more semiconductor thin film photoelectric conversion units and a second electrode stacked in sequence on a substrate at least a surface portion of which is insulated. Further, one photoelectric conversion unit includes an i-type layer sandwiched between a p-type layer and an n-type layer.
A major portion of the thickness of the thin film photoelectric conversion unit is occupied by the i-type layer of a substantially intrinsic semiconductor layer and photoelectric conversion occurs mainly in the i-type layer. Accordingly, it is preferable that the i-type layer as a photoelectric conversion layer has a greater thickness for the purpose of light absorption, though increase of the thickness increases costs and time for deposition of the i-type layer.
The p-type and n-type conductive layers serve to produce a diffusion potential within the photoelectric conversion unit, and magnitude of the diffusion potential affects the value of open-circuit voltage which is one of important properties of the thin film photoelectric conversion device. However, these conductive layers are inactive layers which do not directly contribute to photoelectric conversion. That is, light absorbed by these inactive layers is a loss, which does not contribute to electric power generation. Consequently, it is preferable to minimize the thickness of the p-type and n-type conductive layers as far as they provide a sufficient diffusion potential.
For this reason, regardless of whether p-type and n-type conductivity type layers included in a photoelectric conversion unit or a photoelectric conversion device is amorphous or crystalline, one whose i-type photoelectric conversion layer which occupies a major portion of the conductivity type layer is amorphous is called an amorphous unit or an amorphous photoelectric conversion device, and one whose i-type layer is crystalline is called a crystalline unit or a crystalline photoelectric conversion device.
Currently, a wide variety of materials and forming technologies have been developed for achieving quality required for conductivity type layers included in a photoelectric conversion device. As a material for a conductivity type layer of a silicon photoelectric conversion device, amorphous silicon or its alloy material or crystalline silicon or its alloy material is generally used. Generally, an amorphous silicon material having a wider band gap than that of a photoelectric conversion layer (i-type layer) or a microcrystalline silicon material having a high impurity activation rate is used for a conductivity type layer, with the intention to attain a high photoelectric conversion characteristic while reducing electric and optical losses as small as possible.
A conductive layer of a silicon photoelectric conversion unit is generally formed by a method substantially the same as that for a photoelectric conversion layer (i-type layer) such as a plasma CVD method. The conductive layer is formed from reaction gas which is a mixture of a raw gas containing atoms of silicon and doping gas containing atoms of a conductivity-type determining impurity. In recent years, a modified process of general plasma CVD method has been attempted in order to form the conductivity type layer.
For example, JP-A-06-232429 discloses a plasma doping method in which an i-type layer is once formed by a plasma CVD method and then plasma processing is carried out in an atmosphere containing a mixture of doping gas and a dilution gas such as hydrogen whereby an area near a surface of the i-type layer is changed to a conductivity type layer. Alternatively, JP-A-10-074969 discloses a method for improving crystallinity of a conductivity type layer in which a conductivity type microcrystalline layer is once formed by a plasma CVD method and then plasma processing is carried out in a hydrogen atmosphere. In both of the methods, film deposition by the plasma CVD method and subsequent plasma processing are carried out as continuous processes in a decompression reaction chamber. Thus, a good junction interface and a high quality conductivity type layer can be formed.
In order to enhance a conversion efficiency of a thin film photoelectric conversion device, it is known that two or more photoelectric conversion units are stacked to form a tandem-type thin film photoelectric conversion device. In this case, a front unit including a photoelectric conversion layer having a large band gap (such as of an amorphous silicon or an Si—C alloy) is disposed closer to the light incident side of the photoelectric conversion device, and a rear unit including a photoelectric conversion layer having a small band gap (such as of an Si—Ge alloy) is disposed behind the front unit in sequence. Thus, photoelectric conversion can be performed over a wide wavelength range of incident light, and the conversion efficiency of the entire photoelectric conversion device can be improved. Among such tandem-type thin film photoelectric conversion devices, one including both of an amorphous photoelectric conversion unit and a crystalline photoelectric conversion unit is occasionally called a hybrid thin film solar cell in particular.
For example, a wavelength of light that can be photoelectrically converted by an i-type amorphous silicon ranges to about 800 nm maximum on the long wavelength side, while an i-type crystalline silicon can photoelectrically convert light having a longer wavelength ranging to about 1100 nm. Here, an amorphous silicon photoelectric conversion layer having large light absorption is enough to light absorption in the thickness of about 0.3 μm or less even in the case of a single layer. However, in order to sufficiently absorb light of a longer wavelength also, a crystalline silicon photoelectric conversion layer having a small light absorption coefficient is preferably about 2 to 3 μm thick or above in the case of a single layer. In other words, a crystalline photoelectric conversion layer generally desirably has a large thickness about ten times of that of an amorphous photoelectric conversion layer.
In the tandem-type thin film photoelectric conversion device, respective photoelectric conversion units are desired to be formed under respective optimum conditions. Therefore, the respective photoelectric conversion units may be formed discontinuously by separate deposition apparatuses. Furthermore, in order to enhance flexibility of manufacturing processes of the tandem-type thin film photoelectric conversion device and to improve production efficiency, the respective photoelectric conversion units may be desired to be formed discontinuously by separate deposition apparatuses.
However, the inventors experienced that, when a first photoelectric conversion unit was formed, then a substrate including the unit was once taken out to the air from a deposition apparatus, and a second photoelectric unit was stacked thereafter, characteristics of the resulting tandem-type thin film photoelectric conversion device deteriorate as compared to that of a tandem-type thin film photoelectric conversion device wherein all units were continuously formed without taking a substrate out to the air.